Myocardial contractility, or cardiac contractility, is a term that can be used to describe the performance of cardiac muscle. In other words, cardiac contractility can refer to the intrinsic ability of cardiac muscle fiber to contract at a given fiber length. Changes in the ability of cardiac muscle to produce force during contraction can result from different degrees of binding between myosin and actin filaments, the core components of muscle fiber. The degree of binding that occurs between these filaments can depend on the concentration of calcium ions in cardiac muscle cells, which, in turn, can be controlled by the sympathetic nervous system. Furthermore, cardiac muscle sensitivity to calcium ions (e.g. calcium ion receptor binding affinity) can also affect cardiac contractility.
Contractile reserve (CR), as defined in this document, is the predicted maximum change in cardiac contractility that can be achieved by a patient. In general, patients with a higher CR may achieve greater improvement in cardiac function after cardiac resynchronization therapy (CRT) than patients with a lower CR. Determining an indication of a patient's CR may therefore be used to assess patient status, assess patient response to therapy, and adjust therapy selection.
CR can be measured using a dobutamine stress test. Dobutamine is an inotropic agent, which causes an increase in heart rate and blood pressure similar to the effects of exercise. The dobutamine stress test includes an echocardiogram done at rest and again at peak heart rate after dobutamine has been administered intravenously. A measured change in contractility between rest and peak heart rate can be used as an indication of CR. Other ways of measuring CR include performing an exercise stress test, using various pacing modalities, and using neural therapies.